A Pinaceae tree is one of tree types which have been the most widely planted in the world. In Japan, Pinaceae trees are utilized in industry and forests of Pinus densiflora in undeveloped natural woodlands, forests of Pinus thunbergii along seashores and the like take root as a landscape representing Japan.
Although they are trees which have been widely planted, grafting is a mainstream as a nutritive growing method for pine and no cutting has been generally adopted.
In the production of Pinaceae trees by grafting, there is known a method where a cytokinin which is one of plant growth regulators having an action of promoting a cell division and formation of a shoot is used to thereby effectively collect explanted plants. Thus, as shown in FIG. 1, Pinaceae trees have short branches of 2 to 3 cm length called short shoots at the root of needle leaves, the top of the short leaf has a shoot apex which is a base of buds at the area corresponding to the basement of needle leaf and, usually, although the shoot apex of the short shoot is in a state of dormancy, the cytokinin is made to act thereon in the above method whereby a shoot is induced from the shoot apex of the short shoot and utilized as a shooting (Non-Patent Document 1).
On the other hand, in a method for producing pine trees by cutting, various investigations have been conducted using a current branch which is said to be usually suitable for cutting, but no method for producing a rooted cutting which can be carried out in a practical scale has been known yet. Main reasons therefor would be the following three.    Reasons 1: As the age of Pinaceae trees elapses, the rooting rate is significantly decreased even in the case of a cutting obtained from the current branch.    Reason 2: In the case of a cutting prepared from the current branch of a young tree, although a rooting rate to some extent is expected, numbers of the current branch are small due to the small tree body and sufficient numbers of cuttings cannot be collected.
For example, in ramifications of P. densiflora and P. thunbergii which are Pinaceae trees, only about five lateral buds grow in an axial manner around the new bud once a year in a natural state and, therefore, numbers of the current branches which can be collected as cuttings are limited and, in the case of trees of as young as 3 to 5 years, cuttings can be collected in small numbers of about 20 to 40.    Reason 3: Even when the current branches of young trees are used as cuttings, it is still necessary that a period as long as 3 to 6 months is required until root striking.
Accordingly, although there are Pinaceae trees in Japan having useful characters such as resistance against pine wilt which are running rampant in Japan, no effective production method by means of cutting has been developed whereby supply of pine seedlings has to rely merely upon grafting seedling or actual seedling derived from seeds. However, in a method for producing the seedlings by means of grafting, working steps are complicated and skillfulness is needed while, in the case of actual seedlings, it is not always possible that the good characters of the parents are delivered. Thus, it is difficult to mass-produce seedlings of uniform quality equipped with the above useful characters at a time in a practical scale.
The present applicant has already reported a method for producing a rooted cutting utilizing a photoautotrophically culturing method as a method for producing a rooted cutting of a hardly rooting tree (Patent Document 1), but, even by such a method, it is still difficult to produce cutting of Pinaceae trees in a practical scale.    Patent Document 1: JP-A-2001-186814    Non-Patent Document 1: Wakushima and Yoshioka, “Studies on Growth of Pinus densiflora Resistant to Pine Wood Nematode—Influence of Period and Frequency of Treatment with Spraying of BAP”, Hiroshima-ken Ringyo Shikenjo Research Report, vol. 27, pages 95 to 100 (1993)